It's kind of a weird question, but I've been testing my car so much and I'm wondering if the mousetrap's spring gets looser or less tense because of all the trials, and if it does, how much does it affect the speed? Or is it such a small difference that it doesn't really matter? I don't really want to build a brand new car the day before States like I did for Regionals...
I haven't really noticed anything, but I can't say for sure.
How many trials are you trying? I normally do 2, then if I have any particular problem, I try and fix it before testing again. I generally don't "waste" time repeating trials when I already know the outcome.
Although, some strings do get worn down, so you may want to replace that the night before. I know that with my pin-axle design, the reserve is such a sudden pull on the string, it frays it a lot unless I twist a few strings together.